In persons with congenital nystagmus (CN), rhythmic to-and-fro oscillations of the eyes produce nearly continuous motion of the retinal image. Nevertheless, individuals with CN rarely report the visual world to be moving or smeared. Much, but not all, of the retinal image motion in CN is apparently "canceled" neurologically by extraretinal signals for the nystagmus eye movements, thereby helping to maintain stability and clarity of the visual world. Another mechanism that can contribute to perceptual stability in persons with CN is adaptation to periodic motion, which has been shown in preliminary studies of normal subjects to elevate the threshold for perceiving similar repetitive motion. The FIRST AIM of the proposed research project is to characterize adaptation to periodic motions that simulate common wave forms of CN. A series of short-term experiments using normal adults will determine 1) how adaptation to periodic motion depends on parameters of the adapting motion and the visual stimulus, 2) the extent to which this adaptation generalizes across parameters of motion, 3) the level of visual processing at which adaptation to periodic motion is likely to occur, and 4) the influence of this adaptation on the perception of motion smear. An additional experiment will assess the magnitude and stability of the adaptation produced by long-term exposure to simulated CN wave forms, to assess the likely contribution of this mechanism to perceptual stability in persons with nystagmus. The SECOND AIM of the proposed research is to characterize the rhythmic torsional eye movements that accompany horizontal CN. The torsional component of CN does not conform to Listing's law, which describes how torsional eye position varies with the direction of gaze in normal subjects. Consequently, the characteristics of the torsional movements in CN place constraints on the neural pathways that generate the signals for CN. Two proposed experiments will evaluate the conjugacy of the torsional component of CN and determine whether voluntary saccades in persons with CN are also accompanied by torsional movements that do not conform to Listing's law. Overall, the proposed research will characterize an important adaptational mechanism that can help maintain perceptual stability in persons with nystagmus, and may help to pinpoint the neural structures that underlie CN.